Internal fertilization is a reproduction process that occurs inside the body of the female organism. This type of fertilization is mostly seen in land animals, bony fishes, birds, reptiles, and mammals. For internal fertilization to take place the male sperm has to fuse with the female ovary in the female reproductive tract.

In this article, we will read about the process of internal fertilization, the types and examples of internal fertilization, the difference between internal and external fertilization, and the advantages and disadvantages of internal fertilization.

What is Internal Fertilization?

Internal Fertilization takes place when the male’s sperm is deposited inside the female reproductive system where the egg is fertilized. Different animals have different methods of internal fertilization, in birds it is done by cloacal kiss while in mammals, reptiles, and certain fishes it is done by copulation. Most plants (bryophytes, pteridophytes, gymnosperms, and angiosperms) and higher animals (reptiles, birds, mammals, and terrestrial fungi) undergo internal fertilization.

How does Internal Fertilization work?

Internal fertilization is a complex process that is integral to the reproductive success of many animal species. There are various stages in this process:

Mating and Copulation

Initiation of Mating: The mating process begins with signals that can be visual, chemical (pheromones), auditory, or a combination of the three. The signals play an important role in attracting and selecting potential mates. Many species also engage in courtship behaviour like dance and displaying physical or acoustic demonstrations to attract the mate.

Copulation: During this process, the male sperm is transferred to the female which is done by special reproductive organs like a penis in mammals or an aedeagus in many insects. The copulatory mechanism varies among different species. In mammals, it involves physical penetration whereas in insects and birds, it involves a brief connection for the transfer of the sperm.

Sperm Transportation

After the sperm is deposited into the female reproductive tract it must travel to meet the egg. This process is complex and only a few sperm reach the egg to fertilize it. In mammals, the sperm travels through the cervix into the uterus and reaches the fallopian tube where the eggs are fertilized. In birds, the sperms are stored in special chambers before fertilizing the egg.

• Fertilization: Once the sperm reaches the egg it penetrates the outer layer of the egg and then fertilizes it for this process to take place several complex biochemical and mechanical interactions have to take place.
• Zygote formation: After fertilization, the genetic material combines to form a zygote and it undergoes cellular division to develop into an organism. In mammals, the zygote gets implanted in the uterus and it develops. In oviparous organisms, the zygote develops inside the egg and later hatches after incubation.

Also Read: Fertilization

Types of Internal Fertilization

There are different types of internal fertilization among different organisms that are adapted according to the environment. There are three major types of internal fertilization:

Oviparous

In this process, the fertilization of the sperm and egg takes place inside the female’s body but the development of the embryo takes place in the outside environment when the female lays the egg. The eggs are encased inside a shell and are incubated until it is hatched. This type of fertilization takes place in birds and reptiles.

Viviparous

Viviparous species give birth to young ones and in this type of internal fertilization, the embryo develops inside the female’s body. This involves the placenta which is a special organ that facilitates nutrients and gas exchange between the mother and the developing embryo. Most mammals, including humans and some species of shark exhibit viviparity giving birth to live young ones.

Ovoviviparous

This type is a blend of oviparous and viviparous type of fertilization. Fertilization is internal and the embryo develops inside the female’s body while encased in an egg. In this type, the embryo gets its nutrition from the egg yolk and not from the mother. Certain types of fish like guppies and a few snakes and lizards exhibit this type of fertilization.

Also Read: Difference between Oviparous and Viviparous Animals

Internal Fertilization Examples

Some examples of organisms that undergo internal fertilization are:

Internal Fertilization Fish

When a fish undergoes internal fertilisation, the male inserts sperm into the female’s vaginal entrance via an intermittent organ. Only 500 of the more than 33,000 species of bony fish and basal cartilaginous fish exhibit this mechanism, making fish comparatively rare in general.

Internal Fertilization Mammals

All mammals like humans, dogs, elephants, and whales use internal fertilization. The mammalian reproduction involves the direct deposit of sperm into the female reproductive system during copulation and the fertilization occurs within the uterus or oviducts.

Internal Fertilization Reptiles

Reptiles like lizards, turtles, and snakes use internal fertilization, where the males of these species have hemipenis or a penis that transfers teh sperm into the female reproductive tract.

Internal Fertilization Amphibians

Most amphibians rely on external fertilization some of them like salamanders and caecilians use internal fertilization. The salamanders use spermatophores which are picked up by the females in the cloacal region to fertilize the eggs internally.

Internal Fertilization Birds

Most of the birds engage in internal fertilization but the method differs from mammals as the birds lack external genitalia. The birds perform a “cloacal kiss” where the male and female cloacal touch and allow the transfer of sperm.

Difference between External Fertilization and Internal Fertilization

Internal fertilization vs. external fertilization is given below: